1. Field of the Invention
The invention relates to the monitoring of airborne particulate radioactivity, and in particular to collimation of radiation emitted by particles on a moving web of a filter medium to measure leakage from a reactor coolant system into the containment of a nuclear reactor.
2. Description of Related Art
Safe operation of nuclear power plants requires the prompt detection of leakage of reactor coolant water. The detection and monitoring of leakage of reactor coolant water into the containment area provides information to operators which permits them to take corrective action when a leak may be detrimental to the safety of a facility. Prompt availability of quantitative information can enable operators to act quickly in response to a change in the rate of leakage.
A limited amount of leakage is expected from the reactor coolant system. Valve stem packing glands, circulating pump seals and other equipment cannot be made totally leakproof. The reactor vessel closure seals and safety relief valves should not leak significantly, and any such leakage should be collectible and isolated from the containment atmosphere so as not to mask any potentially serious leak which could occur from a source such as a crack in a pipe.
Uncollected leakage to the containment atmosphere increases the humidity of the containment. The moisture removed from the atmosphere together with any liquid leakage is collected in sumps or tanks where the flow rate can be detected and monitored during plant operation, but maintaining a water inventory balance requires several hours of analysis per day, and does not provide a prompt indication of a change in leakage rate.
Reactor coolant normally contains sources of radiation which, when released to the environment, can be detected by monitoring systems. Reactor coolant radioactivity can be expected to be low during the initial startup of a reactor and for a few weeks thereafter until activated corrosion products have been formed and fission products become available from failed fuel elements. Some leak detection systems are more sensitive than others and are thus better suited for use when coolant radioactivity is low.
Leak detection methods now in use include monitoring of sump-flow and airborne particulate activity, monitoring of condensate flow from air coolers and monitoring of airborne gaseous radioactivity. Most power plants employ several leakage detection methods. In the United States, federal regulations recommend that reactor operators should be able to detect a coolant leakage rate to the containment of one gallon (about 3.79 liters) per minute within one hour of the start of such a leak.
The ability to detect a change in leakage rate promptly can avoid false alarms resulting from belated detection of unexplained leakage and thereby reduce unnecessary and costly shutdowns by permitting early identification of leak sources and prompt corrective action.
The continuous sampling of air from within a reactor containment and detection of airborne particulate radioactivity is an effective way of detecting coolant leakage. One means for detecting particulate radioactivity employs a moving web of filter paper exposed to a flow of air for collection of particles from the air flow. The moving web which has been exposed to the air stream and has collected radioactive particles is caused to pass a detector to measure the radioactivity emitted by particles on the filter paper web. In this application, the term "activity" is sometimes used, for the sake of brevity, to mean the radioactivity of the particles collected on such a web. Changes in the intensity of radiation sensed by the detector correspond to changes in coolant water leakage within the containment from which the sampled air is derived.
The coolant water circulating in a pressurized light water reactor contains, among other things, the radioactive isotope of krypton, Kr.sup.88, which decays by negative beta emission to the radioactive rubidium isotope Rb.sup.88, which also decays by negative beta emission. The half-life of Rb.sup.88 is 17.8 minutes. The relatively short half-life of Rb.sup.88 makes its presence useful as an indicator of coolant leakage, since its decay is much more noticeable than that of such longer lived species as Co.sup.60 and Cs.sup.137 which may also be present in the reactor coolant water.
When coolant water carrying Kr.sup.88 escapes from the coolant system through leakage the krypton is essentially uniformly distributed throughout the volume of gases within the containment vessel. Continuous sampling and monitoring of the containment atmosphere for the presence of the daughter Rb.sup.88 can accordingly reveal the history of leakage within the containment. The present technology for performing this monitoring consists of passing the sampled air through a moving web of filter paper so that Rb.sup.88 is collected by the paper. The moving filter paper web passes a detector such as a scintillometer or geiger-mueller detector for sensing beta particle emission from the web, producing a signal representative at any given time of the total activity of that area of the web then being viewed by the detector. Changes over time in the amount of activity sensed show changes in leakage of coolant water to the containment atmosphere. Conventional display and recording devices are employed to permit plant personnel to monitor changes over time in the amount of detected radiation and thereby respond to signals indicating a change in the rate of coolant leakage. An automatic signal may be activated when the radiation measured or the change in the radiation measured exceeds a predetermined value. This currently employed procedure shows only qualitative differences, and not quantitative differences, because it is not a first order process.
The mathematical processes that have been developed for use in connection with the collection and detection of activity from sampled radioactive atmospheres involve manageable equations which use first order rate constants for all of the removal processes. The major removal processes are radioactive decay (a first order process) and removal by movement of the sample whose activity is being measured. The use of collimation in accordance with the invention makes the activity removal by web movement a first order process with a first order rate constant. The first order rate constants representing radioactive decay and filter web removal from the view of the detector can be readily combined to produce a single first order rate constant.